Flyback converter with output current calibration

ABSTRACT

An output current calibration is disclosed to increase the accuracy and precision of the constant-current mode for a flyback converter.

TECHNICAL FIELD

This application relates to flyback converters, and more particularly toa flyback converter with output current calibration.

BACKGROUND

The charging of a battery for a battery-powered device occurs throughvarious constant-voltage or constant-current modes depending upon thestate of the battery charge. As implied by the name, the chargingvoltage stays constant at some fixed level during a constant-voltagemode whereas the charging current may vary. Conversely, the chargingcurrent stays constant during a constant-current mode whereas thecharging voltage may vary. The proper sequencing and control of theconstant-voltage and constant-current modes is vital with regard tobattery longevity. For example, a smartphone's battery is oftenintegrated and non-removable. If the battery for such a device isdamaged, the entire smartphone must be replaced. It is thus conventionalfor a mobile device to include a battery management circuit thatcontrols the charging voltage and charging current for the battery.

Since the battery management circuit within the mobile device iscontrolling the charging voltage and charging current applied to thebattery, the tolerances for the switching power converter supplyingpower to the mobile device are relaxed. An example charging system isshown in FIG. 1. A switching power converter such as a flyback converter100 converts an input voltage into a regulated output voltage V_outduring a constant-voltage mode of operation. A battery-powered devicesuch as a smartphone 105 includes a battery management circuit 110 thatcontrols the constant-voltage or constant-current charging applied to abattery for powering a system 115 using the output power from flybackconverter 100. For example, in a constant-voltage mode, batterymanagement circuit 110 regulates the constant output voltage fromflyback converter 100 into a constant charging voltage for the battery.Similarly, in a constant-current mode, battery management circuit 110regulates the constant output current from flyback converter 100 into aconstant charging current for the battery. This regulation by batterymanagement circuit 100 provides some tolerance for the regulation inflyback converter 100.

For example, the output voltage and output current tolerance for flybackconverter 100 may be +/−5% as shown in FIG. 2 for the constant-voltageand constant-current modes of operation. For a desired constant-voltagemode of 5 V, the 5% tolerance means that output voltage can actuallyrange from 4.75 V to 5.25V. To maintain regulation duringconstant-voltage operation, a primary-side controller (not illustrated)in flyback converter 100 needs some means of sensing the output voltage.In a primary-only-feedback configuration, the output voltage may besensed through an auxiliary winding (or through the primary winding). Asalso shown in FIG. 2, an analogous tolerance range occurs for the outputcurrent during the constant-current mode of operation. Just like theoutput voltage, a primary-side controller cannot sense the outputcurrent directly but must instead do so indirectly. To indirectly sensethe output current, a primary-side controller may sense the peak currentin a switching cycle through the power switch using a sense resistor.The output current can then be estimated since the output current isproportional to the peak current. In particular, this proportionalitydepends upon a number of factors including the transformer turns ratio,the switching period, the sense resistor resistance, and the transformerreset time.

Such indirect sensing of the output current is adequate if the outputcurrent tolerance is fairly large such as shown in FIG. 2. But portabledevices have been developed in which battery management circuit 100 iseither absent or bypassed in what is denoted herein as a direct-chargesystem. In a direct-charge system, the power converter itself isdirectly charging the portable device's battery. But note that modernsmartphones typically have the battery non-removably integrated into thephone so that if the battery is defective, the entire smartphone becomesdefective. This is especially problematic given the high cost of modernsmartphones. It is thus critical that a power converter such as flybackconverter 100 regulate the constant-voltage and constant-current modeswith considerable precision so that the health of the mobile device'sbattery is preserved. The tolerance for these operating modes is thusreduces in direct-charge systems (e.g., +/−1% of the desired constantcurrent or constant voltage). Despite this reduced tolerance, note thatthere is a substantial tolerance on the component parameters controllingthe proportionality between the peak current and the output current suchas the sense resistance and the transformer reset time. It is thusproblematic for primary-side-regulation of the output current to achievethe necessary tolerance during constant-current operation.

There is thus a need in the art for flyback converters having improvedprimary-side regulation of the output current during constant-currentoperation.

SUMMARY

A flyback converter for direct-charge applications is provided with asecondary-side output current calibration circuit. This calibrationcircuit senses the output current so that the regulation of the powerswitch cycling in constant-current modes of operation may be adjustedresponsive to the sensed output current. But this sensing of the outputcurrent is done relatively infrequently as compared to the power switchcycling frequency so that stability of the control loop for theconstant-current operation is not affected by the output currentcalibration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a conventional mobile device charging system inwhich the mobile device includes a power management circuit.

FIG. 2 is a plot showing the increased tolerance due to the mobiledevice's power management circuit for the constant-voltage andconstant-current modes of operation in the power converter in system ofFIG. 1.

FIG. 3A a diagram of a direct-charge system in which a mobile devicedoes not include a power management circuit and in which a flybackconverter includes a secondary-side output current calibration circuitin accordance with an aspect of the disclosure.

FIG. 3B is a block diagram for the flyback converter of FIG. 3A.

FIG. 4 is plot of the power switching waveform and the peak voltageadjustment waveform for the flyback converter of FIG. 3B.

FIG. 5 is a more detailed illustration of the constant-currentcalibration for the flyback converter of FIG. 3B.

Embodiments of the present disclosure and their advantages are bestunderstood by referring to the detailed description that follows. Itshould be appreciated that like reference numerals are used to identifylike elements illustrated in one or more of the figures.

DETAILED DESCRIPTION

To address the need in the art for improved primary-side regulation ofthe output current during constant-current operation, a secondary-sidecalibration technique is introduced. This calibration is relatively lowfrequency and thus does not disrupt or alter the loop stability for theprimary-side regulation. To better appreciate the advantages of thiscalibration, the factors affecting the accuracy of primary-sideregulation during a constant-current mode of operation for a flybackconverter will first be discussed. The output current of a flybackconverter operating in the discontinuous conduction mode is given by thefollowing Equation (1):

$\begin{matrix}{I_{o} = {\frac{I_{\sec \_ {pk}}}{2}\frac{T_{rst}}{T_{p}}}} & {{Eq}.\mspace{14mu} (1)}\end{matrix}$

where I₀ is the output current, I_(sec_pk) is the secondary peakcurrent, T_(p) is the switching cycle period for the power switch, andT_(rst) is the transformer reset time. The transformer reset time isdefined by the delay between the cycling off for the power switch andthe subsequent ramping of the secondary current ramping from its peak(I_(sec_pk)) to zero. The secondary peak current I_(sec_pk) equals theprimary peak current I_(pk) multiplied by the turns ratio N_(ps) for theflyback's transformer. Since a sense resistor Rs is used to measure theprimary peak current (I_(pk)) by a measurement of the peak voltage(V_(ipk)) across the sense resistor during each switching cycle, therelationship for the output current in Equation (1) may be rewritten asshown in the following Equation (2):

$\begin{matrix}{I_{o} = {\frac{1}{2}N_{ps}\frac{V_{ipk}}{R_{sense}}\frac{T_{rst}}{T_{p}}}} & {{Eq}.\mspace{14mu} (2)}\end{matrix}$

An examination of Equation (2) illustrates the difficulties inmaintaining a precise regulation of the output current. For example, themeasurement of Tp, Trst, Nps, and Rsense are all subject tomanufacturing tolerances, parasitic effects, and resulting uncertainty.A conventional regulation of the output current during aconstant-current mode of operation will thus fail to satisfy thestringent requirements for a direct-charge system. From Equation (2), itmay be seen that during constant-current mode, is conventional to setthe peak voltage V_(ipk) using the following Equation (3):

$\begin{matrix}{V_{ipk} = {k_{CC}\frac{T_{p}}{T_{rst}}}} & {{Eq}\mspace{14mu} (3)}\end{matrix}$

Where k_(CC) is a proportionality constant that is conventionallystatic. By comparing Equations (2) and (3), it may be seen that bycontrolling the peak voltage V_(ipk) according to Equation (3) keeps theoutput current constant as is appropriate for a constant-current mode ofoperation. The output current as a function of the proportionalityconstant k_(CC) is given by the following Equation (4):

$\begin{matrix}{I_{o} = {\frac{1}{2}N_{ps}\frac{k_{CC}}{R_{sense}}\eta_{x}}} & {{Eq}.\mspace{14mu} (4)}\end{matrix}$

where ηx is the transformer conversion ratio. The switching period T_(p)is typically constant (although it may be subject to dithering) so thatthe transformer reset time T_(rst) controls the bulk of the adjustmenton a cycle-to-cycle basis for the peak voltage. In a conventionalprimary-only regulation of the constant output current, the factors Tpand Trst in one switching cycle are applied to the subsequent switchingcycle to determine the desired peak voltage. The voltage across thesense resistor is then compared to the desired peak voltage to determinethe switch on-time by cycling off the power switch once the voltageacross the sense resistor reaches the desired peak voltage. But thisconventional primary-side regulation of the output current isproblematic with regard to providing the desired degree of precision andtolerance such as required for direct-charge systems.

To provide increased regulation precision and tighter tolerances for theconstant-current mode operation, a secondary-side calibration circuit isprovided that directly measures the output current under known operatingconditions. For example, the secondary-side calibration circuit may usea secondary-side sense resistor. Since the resistance is known for thesecondary-side sense resistor, the secondary-side calibration circuitmay measure the output current by detecting the voltage across thesecondary-side sense resistor and dividing by the resistance of thesecondary-side sense resistor as governed by Ohm's law. Note that thereis a proportionality between the proportionality constant k_(CC) ofEquation (3) and the desired value for the output current as governed byEquation (4). The secondary-side calibration circuit thus commands theprimary-side controller to adjust its proportionality constant k_(CC)based upon the difference between the desired output current and themeasured output current. In this fashion, the constant-current moderegulation precision is advantageously improved. Despite the use of asecondary-side calibration circuit, the primary-side regulation loop isunaffected since the frequency of calibration is relatively low. Incontrast, the switching frequency for the power switch is oftenrelatively high (e.g., 50 KHz or higher). Since the calibration isperformed relatively infrequently, it has no real effect on theprimary-side regulation of the output current. There are thus nostability issues raised by the inclusion of the secondary-sidecalibration circuit.

An example flyback converter 300 with secondary-side output currentcalibration is shown in FIG. 3A. The precision provided by thesecondary-side output current calibration enables flyback converter 300to safely power a directly charge a battery for powering a system 302 ina mobile device 301. In this direct-charge configuration, there is nopower management circuit that controls the charging of the battery inmobile device 301. Instead, it is flyback converter 300 that mustregulate the constant-voltage and constant-current charging modes forthe battery. Flyback converter 300 is shown in more detail in FIG. 3B. Aprimary-side controller 305 controls the switching of a power switchtransistor S1 to regulate operation in either a constant-voltage or aconstant-current mode of operation. During constant-voltage regulation,controller 305 samples an output voltage Vout indirectly by sampling afeedback voltage (VFB) on an auxiliary winding 310 of a transformer T1.The feedback voltage is filtered in a loop filter 315 and compared to areference voltage (V_REF) at an error amplifier (EA) to produce an errorvoltage. Depending upon the error voltage and the desired outputvoltage, controller 305 adjusts the modulation of the cycling of thepower switch transistor such as through pulse-width modulation or pulsefrequency modulation to keep the output voltage at the desired levelduring constant-voltage operation. But depending upon the charging stateof a battery (not illustrated) being charged in a mobile device,controller 305 may not be able to adjust the output voltage to thedesired level. Controller 305 then switches to a constant-current modeof operation as represented by a CC mode controller 320 that is shownseparately from controller 305 for illustration purposes since it wouldtypically be integrated with controller 305.

In the constant-current mode of operation, controller 320 monitors thepeak voltage Vipk that develops across a sense resistor Rsense that isin series with the power switch transistor S1, which in turn is inseries with a primary winding of transformer T1. During the on-time foreach power switch cycle, a rectified input voltage V_IN causes amagnetizing current to flow through the power switch transistor S1 andthus through the sense resistor. Once the peak voltage Vipk reaches adesired level, controller 305 switches off the power switch transistorS1. A peak secondary current then flows through a secondary winding fortransformer D1 to produce the output current I_OUT that flows to theload (not illustrated). The output current charges a smoothing capacitorC2 to develop the output voltage. An output diode D1 rectifies thesecondary current although it will be appreciated that suchrectification may be performed using a synchronous rectifier switch asknown in the synchronous rectification arts.

Controller 320 can only indirectly control the output current such asthrough the use of Equation (3) above since controller 320 is isolatedthrough transformer T1 from the secondary side of flyback converter 300.In a traditional use of Equation (3), the proportionality constantk_(CC) is constant. But a Vipk calibration circuit 325 in flybackconverter 300 directly measures the output current so that theproportionality constant k_(CC) may be adjusted to keep the outputcurrent at the desired level. In this fashion, the inaccuraciesresulting from parasitic effects and component tolerances such as forthe sense resistor are reduced so that the output current may beregulated in constant-current mode according to the tight tolerances(e.g., +/−1%) demanded by direct-charge applications. The measurement ofthe output current by Vipk calibration circuit 325 occurs on thesecondary-side of transformer T1. But this measurement cannot bedirectly communicated to the primary-side of transformer T1 such asthrough a wire or lead because the isolation between the grounds on theprimary and secondary sides of transformer T1 would be destroyed. Thus,Vipk calibration circuit 325 communicates the necessary calibrationinformation through an isolating channel such an optoisolator 330. Inalternative embodiments, other types of isolating channels may be usedsuch as a capacitor or a use of transformer T1 itself.

Given the calibration information regarding the measured output current,controller 320 alters the proportionality constant k_(CC) accordingly.In this fashion, the level for Vipk is altered from what a conventionalapplication of Equation (3) would provide. Some example waveforms forthe power switch cycling and the sense resistor voltage are shown inFIG. 4. For each on-time of the power switch transistor, the senseresistor voltage rises from ground to Vipk, whereupon the power switchtransistor is cycled off. Referring again to Equation (3), the switchingperiod Tp is substantially constant so it is the transformer reset timeTrst that controls the conventional cycle-to-cycle adjustment of Vipk.But the calibration of the proportionality constant k_(CC) raises orlowers Vipk accordingly so that the output current is more tightlyregulated.

Additional details for the proportionality constant calibration inflyback converter 300 are shown in FIG. 5. To directly measure theoutput current, a secondary-side sense resistor is inserted into theground path return for output voltage. The output current lout isdelivered to the load (not illustrated) during constant-currentoperation and returns along the ground path to develop a voltage acrossthe secondary-side sense resistor that is proportional to the outputcurrent. A current sensing circuit and analog-to-digital converter (ADC)500 in secondary-side output calibration circuit 325 senses the outputcurrent lout by sensing the sense resistor voltage and proportionallyconverting it into a digital value representative of the output current.A constant-current mode detection and filtering circuit 501 detectswhether the load is being driven in a constant-current mode by, forexample, comparing the output current to an output current limit. Shouldregulation of the output voltage drive the output current to reach anoutput current limit, flyback converter 300 can no longer operate in aconstant-voltage mode but must instead operate in a constant-currentmode so that that the output current does not exceed the output currentlimit. In addition, constant-current mode detection and filteringcircuit filters the output current calibration so that the outputcurrent calibration is done relatively infrequently as compared to thepower switch cycling frequency. In this fashion, the gain and phasemargin for the constant-current mode of operation is not affected by theoutput current calibration.

During the constant-current mode of operation, controllers 320 and 305(are regulating the cycling of the power switch transistor to keep theoutput current at a desired level, which is designated herein as Iset.Calibration circuit 325 includes an adder 505 that forms the differencebetween the measured output current and Iset. This difference is thensubtracted from Iset in another adder 510 to form a calibrated desiredoutput current (designated herein as iset). This adjusted desired outputcurrent maps to an adjusted proportionality constant k_(CC) as definedby an iset-to-k_(CC) conversion circuit 520 that is then communicatedthrough an isolating communication channel such as optoisolator 330 tothe primary side of flyback converter 300. Should the output currentcalibration indicate that the measured output current is greater thanthe desired level, the iset-to-k_(CC) conversion lowers k_(CC) from itsdefault value that would otherwise be used for the achievingconstant-current operation at the desired level. Conversely, should theoutput current calibration indicate that the measured output current isless than the desired level, the iset-to-k_(CC) conversion increasesk_(CC) from its default value that would otherwise be used for theachieving constant-current operation at the desired level. It will beappreciated that adders 505 and 510, conversion circuit 520, and theconstant-current detection and filtering circuit 501 may be performedeither on the secondary side or the primary side of the transformer.

Constant-current mode controller 320 uses the transformer reset timeTrst as determined by sensing the reflected voltage on auxiliary winding310 and forms a digital version (Vipk_d) of the desired peak voltageusing Equation (3) with the adjusted proportionality constant k_(CC). Adigital-to-analog converter (DAC) converts the digital peak voltage intothe analog peak voltage Vipk so that it compared to the primary-sidesense resistor voltage in a comparator 515. When the sense resistorvoltage reaches the peak voltage Vipk, the comparator output signaltransitions to a binary one level (e.g, a power supply voltage) totrigger controller 305 to shut off the power switch transistor so as toterminate its on-time in the current power switch cycle. Controller 305may regulate this power switch cycling in either a pulse-widthmodulation (PWM) or a pulse frequency modulation (PFM) mode ofoperation. But note that the resulting regulation of the output currentis now much more tightly controlled due to the adjustment with regard tothe sensing of the output current.

As those of some skill in this art will by now appreciate and dependingon the particular application at hand, many modifications, substitutionsand variations can be made in and to the materials, apparatus,configurations and methods of use of the devices of the presentdisclosure without departing from the spirit and scope thereof. In lightof this, the scope of the present disclosure should not be limited tothat of the particular embodiments illustrated and described herein, asthey are merely by way of some examples thereof, but rather, should befully commensurate with that of the claims appended hereafter and theirfunctional equivalents.

We claim:
 1. A circuit for a flyback converter, comprising: a peakvoltage determination circuit configured to determine a desired peakvoltage for a current switching cycle of a power switch transistorresponsive to a transformer reset time and an adjusted proportionalityconstant, wherein the adjusted proportionality constant is adjustedresponsive to a determination of an output current for the flybackconverter; a comparator configured to compare the desired peak voltageto a primary-side sense resistor voltage; and a power switch controllerconfigured to shut off a power switch transistor responsive to an outputsignal from the comparator indicating that the primary-side senseresistor voltage equals the desired peak voltage.
 2. The circuit ofclaim 1, wherein the peak voltage determination circuit is furtherconfigured to determine the desired peak voltage responsive to aswitching period for a cycling of the power switch transistor.
 3. Thecircuit of claim 1, wherein the peak voltage determination circuit andthe power switch controller are both implemented within amicrocontroller.
 4. The circuit of claim 1, further comprising: acurrent sense circuit configured to sense a secondary-side senseresistor voltage; and an analog-to-digital converter configured toconvert the secondary-side sense resistor voltage into a digital signal.5. The circuit of claim 4, further comprising: a first adder configuredto determine a difference between the digital signal and a desired levelfor the output current.
 6. The circuit of claim 5, further comprising: asecond adder configured to determine a difference between the desiredlevel for the output current and the difference from the first adder toprovide an adjusted output current level.
 7. The circuit of claim 6,further comprising: a conversion circuit configured to convert theadjusted output current level into the adjusted proportionalityconstant.
 8. The circuit of claim 1, further comprising an isolatedcommunication channel, wherein the peak voltage determination circuit isfurther configured to receive the adjusted proportionality constant fromthe isolated communication channel.
 9. The circuit of claim 8, whereinthe isolated communication channel comprises an optoisolator.
 10. Thecircuit of claim 8, wherein the isolated communication channel comprisesa transformer for the flyback converter.
 11. The circuit of claim 1,wherein the peak voltage determination circuit is further configured todetermine the desired peak voltage during a constant-current mode ofoperation for the flyback converter.
 12. A method for calibrating aflyback converter during a constant-current mode of operation,comprising: sensing an output current for the flyback converter toprovide a sensed output current; adjusting a proportionality constantresponsive to a difference between a desired output current and thesensed output current to provide an adjusted proportionality constant;and determining a peak voltage for a primary-side sense resistor duringa current cycle of a power switch transistor responsive to the adjustedproportionality constant.
 13. The method of claim 12, furthercomprising: shutting off the power switch transistor during its currentcycle responsive to a voltage for the primary-side sense resistorequaling the peak voltage.
 14. The method of claim 12, wherein thedetermining of the peak voltage is further responsive to a transformerreset time.
 15. The method of claim 12, wherein the determining of thepeak voltage is further responsive to a period for the cycling of thepower switch transistor.
 16. The method of claim 12, wherein the sensingof the output current comprises sensing a voltage for a secondary-sidesense resistor to provide a sensed voltage.
 17. The method of claim 16,further comprising digitizing the sensed voltage to provide a digitizedsensed voltage.
 18. The method of claim 17, further comprisingdetermining a difference between the desired output current and thedigitized sensed voltage to provide a first difference.
 19. The methodof claim 18, further comprising determining a difference between thedesired output current and the first difference to provide a seconddifference, and wherein the adjusting of the proportionality constantresponsive to the difference between a desired output current and thesensed output current comprises adjusting the proportionality constantresponsive to the second difference.